Super Capacitor Reference Design of LED Flash for High Resolution Camera Phone

Mobile phones are becoming the ultimate integrated portable consumer electronics platform. Its performance includes capturing high-quality images, Wi Fi network access, crisp audio, longer call time, and longer battery life. However, a major design challenge is also emerging. In order to adapt to highly complex mobile applications, mobile phone batteries still need a lot of effort to provide sufficient peak power, which promotes the demand for circuits that can provide the required power for high-performance operation. This circuit can store high current in a short time without overloading the battery.

For advanced camera phone manufacturers, the most important challenge is to provide the high peak current required for high brightness camera flash. As the resolution of camera phones increases to 3 million pixels or more, the luminous flux required to produce high-quality images has also increased sharply. In order to match the photo quality of the digital camera, the LED flash must be driven with a current of up to 2a, or the xenon flash tube must be charged above 330V. Other applications of mobile phones (including RF power amplifier, GPS navigation, Internet access, music and video) may also exceed the supply capacity of battery current.

Design challenges camera phones require a high brightness flash to produce fully exposed pictures in moderate to low light conditions. Designers can choose LED or xenon flash tube, but both of them have corresponding challenges.

High current LED flash requires 4 times the power provided by the battery to produce the brightness required for high-resolution images. In order to overcome the power limitation, some camera phones have adopted longer flash exposure time to compensate for the lack of luminous flux, which will lead to the blurring of the picture.

Xenon flash tube can provide good illumination, but its flash exposure time is very short, so it can not be used for video capture / movie mode function. The electrolytic storage capacitor required by it is too large for the thin design, and the working voltage is very high. It takes a long time to be fully charged between two flashes. It can not be used in other applications requiring peak power in mobile phones.

One of the ways to solve the problem of providing 1 2A driving current for each LED flash is to use a capacitor to store the current and quickly supply power without shunting the main battery. However, if the traditional capacitor is used to store large current, either a very large capacity capacitor or multiple medium capacity capacitors connected in parallel are required. For the portable system with limited space, the more practical solution is to use super capacitor with very high capacitance. By using a supercapacitor, designers can provide the required high current for these short duration events and recharge them through the battery between these events. In order to support the battery, designers can add a very thin super capacitor, which can meet the peak power demand of the mobile phone without sacrificing the thin mobile phone design (such as providing flash for photography, audio and video, wireless transmission and GPS result reading). It also allows designers to optimally adjust the size of batteries and power circuits by meeting only the average power consumption rather than the peak power consumption, so as to reduce the board area of the system.

Design a super capacitor what is super capacitor (SC)? Like any capacitor, a supercapacitor is basically composed of two parallel conductive plates separated by an insulating material called dielectric. The capacitance of the capacitor is directly proportional to the area of the conductive plate and inversely proportional to the thickness of the dielectric. Manufacturers developing supercapacitors have achieved higher capacitance values by using porous carbon materials to manufacture conductive plates to maximize the surface area and using electrolyte as thin as molecules as dielectric to minimize the distance between the two conductive plates.

Using this method, a capacitor with a capacitance of 16mf 2.3f can be manufactured. These capacitors can be equivalent to very low internal resistance or ESR (equivalent series resistance), which allows them to provide peak current pulses while minimizing the output voltage. By providing very high capacitance with relatively small overall dimensions, these super capacitors reduce the system's demand for PCB area. They can be manufactured in any size and shape, can be recharged in seconds, can extend battery life by five times, and allow designers to use smaller, lighter and cheaper batteries.

Inherent challenges, however, this low ESR poses a problem for designers during charging. In any system, the capacitor is vented before initial use. Then, when the supply voltage is applied, the supercapacitor looks like a low resistance. If the current is not controlled or limited, it will lead to a huge peak current. Therefore, the designer must implement some peak current limiting measures to ensure that the battery will not run out immediately. Any such type of circuit usually also requires short circuit, overvoltage and overcurrent protection.

The challenge for designers is how to efficiently connect the battery, DC / DC converter and super capacitor, so as to not only limit the peak charging current of super capacitor, but also continuously recharge the super capacitor between loads. LED flash drivers that can meet the requirements of super capacitor charging have now appeared in the market. It can make the work of designers easier and save PCB space, cost and time to market. The LED flash of digital camera needs 1 2A current to flash continuously for 120ms.

Super capacitors can be used to store the necessary current and supply power quickly without shunting the main battery. When working with the battery, the supercapacitor releases its stored current during the peak load and recharges between the two peak loads. Compared with pure battery powered equipment, the power supply system with super capacitor can provide up to twice the power and prolong the service life of the battery. Obviously, whenever designers use supercapacitors, they must limit peak currents. In addition, when the voltage falls below the working voltage of the LED flash lamp, the super capacitor needs to be recharged. When the supercapacitor is fully charged, it must be disconnected from the charging source. In addition, it also needs short-circuit protection, source overvoltage protection and overcurrent protection.

Benefits of super capacitor super capacitor can recharge in a few seconds for a period of more than 500K, and store electric energy in the electrostatic field. Since only a large load current can reduce the voltage too low when fully charged, the use of supercapacitors also reduces ESR and impedance.

Supercapacitors can be manufactured in any size and shape, whether flat or small. The super capacitor also has a long service life (10 12 years). Unlike batteries, they have a non-destructive open circuit (high ESR) failure mode. If an excessive voltage is applied to the device, the only consequence is a slight increase in ESR and eventually evolves to an open circuit state. There will be no fire, smoke or explosion in the whole process.

Design solution super capacitor powered LED flash unit can drive high current LED, which provides many times the flash brightness of standard battery powered LED flash unit, or the duration exceeds xenon lamp. As shown in Figure 1, aat1282 contains a boost converter, which is used to raise the input voltage of 3.2 4.2V battery to a stable 5.5V. If the battery voltage is 3.5V and the boost converter efficiency is 90%, the battery needs to provide more than 3A current to maintain a 2A flash pulse. This will either cause the battery protection circuit to turn off the battery, or cause a low voltage shutdown, while the battery still has a large amount of power.

This solution also provides some flash management functions, such as movie mode and super capacitor charging. The solution controls and regulates the current from the mobile phone battery source, increases the battery voltage and manages the charging of the super capacitor to control and provide the high current required by the LED flash in the terminal application.

Figure 1 power supply for LED with super capacitor

Fig. 2 detailed circuit principle of aat1282

In order to better achieve this goal, aat1282 has a built-in circuit to prevent excessive peak current impact during startup, a fixed 800mA input current limiter, and a load disconnection circuit after the super capacitor is fully charged. Its output voltage is limited by the internal overvoltage protection circuit, which can prevent the converter and super capacitor from being damaged by an open LED (under open circuit conditions).

During the open circuit state, the output voltage rises and reaches 5.5V (typical value), and the overvoltage protection circuit closes the switch circuit to prevent the output voltage from rising further. Once the open circuit state is released, the switch circuit will resume operation immediately. The controller returns to normal operation and maintains an average output voltage. An industry standard I2C serial digital input interface enables / locks LEDs and sets the movie mode current with up to 16 movie mode settings for lower light output requirements.

A detailed schematic diagram (see Figure 2) shows that only a small number of components are required around the supercapacitor. A 0.55f, 85m Ω super capacitor can provide 9W led burst power when used with aat1282 LED flash driver. In order to achieve high brightness, the driving current of LED flash lamp is between 1 2A. The forward voltage (VF) on the LED can rise to 4.8V. If we include the 200mV overhead of the current control circuit, you can easily see how the total load voltage rises to 5V during the flash event, which also proves that the 5.5V output voltage is necessary.

Fig. 3 LED flash test results under different driving conditions

Figure 4 digital control of flash function and movie mode options

Figure 3 shows the test results using two LED flashers driven by 1A current and one LED flash driven by 2A current. As can be seen, the supercapacitor can easily provide the current required for the flash to last for 120ms, while keeping the power supply voltage sufficiently higher than the VF voltage of the LED. Between two flashes, the supercapacitor recharges at a slower rate, and the charging time is set externally, which can be optimized for different battery sizes and chemical principles. Figure 4 shows the digital control of the flash function and movie mode options.

Conclusion supercapacitors are rarely used in portable systems. Their applications are usually limited to backup or standby. These functions use relatively low current and require considerable charging time. By integrating the latest boost converter and supercapacitor, designers can now create compact solutions.